MUSEUM OF COMPARATIVE ZOOLOGY, 33 
seen any such circular groove surrounding the branch as he figures. In 
all cases the two layers of the body wall form a circular fold, in which, 
however, there is never, even at the earliest stages, a space between the 
ectodermal layers, nor any infolding of the cuticula as Korotneff (75, p. 
369), according to Hoyer’s rather incomplete abstract, maintains (Plate 
V. Fig. 47). When the circular fold has advanced until only a small pore 
remains, by which the cavities of the older and younger individuals are 
kept in communication, the mesodermal cells at the angle of the fold 
begin to undergo a metamorphosis both in form and histological charac- 
ter. In the first place they become much elongated and extremely 
attenuated, passing from one surface of the septum to the other, and 
forming the lips of the pore. In the second place their plasma becomes 
first deeply stainable, and later, in addition, homogeneous and highly 
refractive. These metamorphosed cells form what may be called the 
teeth of the plate. They are derived wholly from mesoderm. 
The cells in the upper mesodermal layer next increase rapidly in 
number and size, and the number of teeth is also augmented (Plate V. 
Fig. 48). The metamorphosis of the cells extends still farther away 
from the communication pore, and involves the lower mesodermal layer ; 
but, apparently, each cell of the latter is metamorphosed only to a 
slight depth within its cell wall (Fig. 51), whereas in each of the upper 
cells the ends which project into the communication pore are modified 
through and through (Fig. 46). At a later stage (Fig. 49) the meta- 
morphosed part of the cell seems quite sharply cut off from the active 
part, and the slits between the metamorphosed teeth are considerably 
reduced. Nevertheless, I believe a transfer of fluids may still occur 
between them, for even in the adult communication plate one can trace 
continuous lumina when the cells are by accident torn off from the 
“teeth” which they have produced. It is important to note that the nu- 
clei are not destroyed in the cell metamorphosis. Some lie above, others 
below the pore, and become deeply stainable. The ectodermal layers of 
the communication plate secrete a cuticula between them, This is thin- 
ner than that of the body wall, and does not extend, of course, to the 
centre of the communication plate, but ends in a thickened ring, whose 
diameter is about one tenth the diameter of the plate, or, absolutely, 
about 9.4 pt 
1 Reichert (70, p. 267) first carefully described the Rosettenplate of Cteno- 
stomes in Zoöbotryon, and the organ in Paludicella must be regarded as homologous 
with it, The central circular hole in the cuticula of Zoöbotryon is from 7 to 10 u 
in diameter, and from one ninth to one seventh that of the entire plate. Similar 
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